Inspection substrate and an inspection method

ABSTRACT

An inspection substrate for inspecting a component of an apparatus for processing production substrates, the inspection substrate has: a body having dimensions similar to the production substrates so that the inspection substrate is compatible with the apparatus; an illumination device embedded in the body, the illumination device configured to emit radiation toward a target area of the component of the apparatus; an imaging device embedded in the body, the imaging device configured to detect radiation scattered at the target area and generate an image from the detected radiation, wherein the illumination device is configured to emit the radiation such that radiation that is specularly reflected at the target area does not contribute to the image generated by the imaging device.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the U.S. national phase entry of PCT patentapplication no. PCT/EP2017/064660, which was filed on Jun. 15, 2017,which claims the benefit of priority of European patent application no.16177745.3, which was filed on Jul. 4, 2016, and which is incorporatedherein in its entirety by reference.

FIELD

The present invention relates to an inspection substrate and aninspection method for use in a lithographic apparatus, metrologyapparatus or a process apparatus, for example.

BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.comprising part of, one, or several dies) on a substrate (e.g. a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned.

Immersion techniques have been introduced into lithographic systems toenable improved resolution of smaller features. In an immersionlithographic apparatus, a liquid layer of a liquid having a relativelyhigh refractive index is interposed in a space between a projectionsystem of the apparatus (through which the patterned beam is projectedtowards the substrate) and the substrate. The liquid covers at last thepart of the wafer under the final lens element of the projection system.Thus, at least the portion of the substrate undergoing exposure isimmersed in the liquid. The effect of the immersion liquid is to enableimaging of smaller features since the exposure radiation will have ashorter wavelength in the liquid than gas. (The effect of the liquid mayalso be regarded as increasing the effective numerical aperture (NA) ofthe system and also increasing the depth of focus.)

In commercial immersion lithography, the liquid is water. Typically thewater is distilled water of high purity, such as Ultra-Pure Water (UPW)which is commonly used in semiconductor fabrication plants. In animmersion system, the UPW is often purified and it may undergoadditional treatment steps before supply to the immersion space asimmersion liquid. Other liquids with a high refractive index can be usedbesides water as the immersion liquid, for example: a hydrocarbon, suchas a fluorohydrocarbon; and/or an aqueous solution. Further, otherfluids besides liquid have been envisaged for use in immersionlithography.

In this specification, reference will be made in the description tolocalized immersion in which the immersion liquid is confined, in use,to the space between the final lens element and a surface facing thefinal element. The facing surface is a surface of substrate or a surfaceof the supporting stage (or substrate table) that is co-planar with thesubstrate surface. (Please note that reference in the following text tosurface of the substrate W also refers in addition or in the alternativeto a surface of the substrate table, unless expressly stated otherwise;and vice versa.) A fluid handling structure present between theprojection system and the stage is used to confine the immersion to theimmersion space. The space filled by liquid is smaller in plan than thetop surface of the substrate and the space remains substantiallystationary relative to the projection system while the substrate andsubstrate stage move underneath.

The fluid handling structure is a structure which supplies liquid to theimmersion space, removes the liquid from the space and thereby confinesliquid to the immersion space. It includes features which are a part ofa fluid supply system. The arrangement disclosed in PCT patentapplication publication no. WO 99/49504 is an early fluid handlingstructure comprising pipes which either supply or recover liquid fromthe space and which operate depending on the relative motion of thestage beneath the projection system. In more recent designs of fluidhandling structure extends along at least a part of a boundary of thespace between the final element of the projection system and thesubstrate table 60 or substrate W, so as to in part define the space.

A lithographic apparatus is a complex apparatus and most of its criticalparts have to be operated under very controlled environments, withhigher contamination specifications than standard for cleanrooms. If theapparatus has to be opened up for maintenance or inspection, timeconsuming processes such as decontamination and start-up may need to betaken before the apparatus can be returned to service. It is desirablethat any downtime of the apparatus be minimized as far as possible sincethis reduces the productivity of the apparatus and increases cost ofownership.

SUMMARY

It is desirable, for example, to provide means to enable critical partsof the apparatus to be inspected with minimum downtime.

According to an aspect, there is provided an inspection substrate forinspecting a component of an apparatus for processing productionsubstrates, the inspection substrate comprising:

a body having dimensions similar to the production substrates so thatthe inspection substrate is compatible with the apparatus;

an illumination device embedded in the body, the illumination deviceconfigured to emit radiation toward target area of the component of theapparatus;

an imaging device embedded in the body, the imaging device configured todetect radiation scattered at the target area and generate an image fromthe detected radiation;

wherein the illumination device is configured to emit the radiation suchthat radiation that is specularly reflected at the target area does notcontribute to the image generated by the imaging device.

According to an aspect, there is provided a method of inspecting acomponent of an apparatus for processing production substrates, themethod comprising:

loading into the apparatus an inspection substrate as described above;

scanning the inspection substrate proximate the component whilstoperating the sensor to generate inspection information relating to aparameter of the component; and

storing the inspection information in the storage device.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 schematically depicts a lithographic apparatus;

FIG. 2 schematically depicts a immersion liquid confinement structurefor use in a lithographic projection apparatus;

FIG. 3 is a side cross sectional view that schematically depicts afurther immersion liquid supply system according to an embodiment;

FIG. 4 depicts the underside of a part of an immersion liquidconfinement structure for use in a lithographic projection apparatus;

FIG. 5 depicts an inspection substrate according to an embodiment of theinvention;

FIG. 6 depicts an inspection substrate according to an embodiment of thepresent invention;

FIG. 7 depicts an arrangement of fields of view of an imaging device ofan embodiment of the present invention;

FIG. 8 is a flow diagram of a method according to the invention;

FIG. 9 depicts in cross-section an illumination arrangement in anembodiment of the invention indicating light paths that illuminate aninspection target, directly and indirectly;

FIG. 10 depicts in cross-section an illumination arrangement in anembodiment of the invention indicating light paths that reach an imagingdevice without illuminating an inspection target;

FIG. 11 depicts in cross-section an illumination arrangement in anembodiment of the invention indicating light paths that reach theimaging device after multiple reflections without illuminating aninspection target;

FIG. 12 depicts an image resulting from the illumination arrangement ofFIGS. 9, 10 and 11;

FIG. 13 depicts in perspective an illumination arrangement of anembodiment of the invention;

FIG. 14 depicts in cross-section a part, including the lens of theimaging device of the illumination arrangement of FIG. 13; and

FIG. 15 depicts the effect of a double stop.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic apparatus in which anembodiment of the invention can be used. The apparatus includes anillumination system (illuminator) IL configured to condition a radiationbeam B (e.g. UV radiation or any other suitable radiation), a masksupport structure (e.g. a mask table) MT constructed to support apatterning device (e.g. a mask) MA and connected to a first positioningdevice PM configured to accurately position the patterning device inaccordance with certain parameters. The apparatus also includes asubstrate table (e.g. a wafer table) 60 or “substrate support”constructed to hold a substrate (e.g. a resist coated wafer) W andconnected to a second positioning device PW configured to accuratelyposition the substrate in accordance with certain parameters. Theapparatus further includes a projection system (e.g. a refractiveprojection lens system) PS configured to project a pattern imparted tothe radiation beam B by patterning device MA onto a target portion C(e.g. including one or more dies) of the substrate W.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The mask support structure supports, i.e. bears the weight of, thepatterning device. It holds the patterning device in a manner thatdepends on the orientation of the patterning device, the design of thelithographic apparatus, and other conditions, such as for examplewhether or not the patterning device is held in a vacuum environment.The mask support structure can use mechanical, vacuum, electrostatic orother clamping techniques to hold the patterning device. The masksupport structure may be a frame or a table, for example, which may befixed or movable as required. The mask support structure may ensure thatthe patterning device is at a desired position, for example with respectto the projection system. Any use of the terms “reticle” or “mask”herein may be considered synonymous with the more general term“patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section so as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam which is reflected by the mirrormatrix.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the moregeneral term “projection system”.

As here depicted, the apparatus is of a transmissive type (e.g.employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g. employing a programmable mirror array of a type asreferred to above, or employing a reflective mask).

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables or “substrate supports” (and/or two or more masktables or “mask supports”). In such “multiple stage” machines theadditional tables or supports may be used in parallel, or preparatorysteps may be carried out on one or more tables or supports while one ormore other tables or supports are being used for exposure.

The lithographic apparatus may also be of a type wherein at least aportion of the substrate may be covered by a liquid having a relativelyhigh refractive index, e.g. water, so as to fill a space between theprojection system and the substrate. An immersion liquid may also beapplied to other spaces in the lithographic apparatus, for example,between the mask and the projection system Immersion techniques can beused to increase the numerical aperture of projection systems. The term“immersion” as used herein does not mean that a structure, such as asubstrate, must be submerged in liquid, but rather only means that aliquid is located between the projection system and the substrate duringexposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery system BDincluding, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus, for example when the source is a mercury lamp.The source SO and the illuminator IL, together with the beam deliverysystem BD if required, may be referred to as a radiation system.

The illuminator IL may include an adjuster AD configured to adjust theangular intensity distribution of the radiation beam. Generally, atleast the outer and/or inner radial extent (commonly referred to asσ-outer and σ-inner, respectively) of the intensity distribution in apupil plane of the illuminator can be adjusted. In addition, theilluminator IL may include various other components, such as anintegrator IN and a condenser CO. The illuminator may be used tocondition the radiation beam, to have a desired uniformity and intensitydistribution in its cross section.

The radiation beam B is incident on the patterning device (e.g., maskMA), which is held on the mask support structure (e.g., mask table MT),and is patterned by the patterning device. Having traversed the mask MA,the radiation beam B passes through the projection system PS, whichfocuses the beam onto a target portion C of the substrate W. With theaid of the second positioning device PW and position sensor IF (e.g. aninterferometric device, linear encoder or capacitive sensor), thesubstrate table 60 can be moved accurately, e.g. so as to positiondifferent target portions C in the path of the radiation beam B.Similarly, the first positioning device PM and another position sensor(which is not explicitly depicted in FIG. 1) can be used to accuratelyposition the mask MA with respect to the path of the radiation beam B,e.g. after mechanical retrieval from a mask library, or during a scan.In general, movement of the mask table MT may be realized with the aidof a long-stroke module (coarse positioning) and a short-stroke module(fine positioning), which form part of the first positioning device PM.Similarly, movement of the substrate table 60 or “substrate support” maybe realized using a long-stroke module and a short-stroke module, whichform part of the second positioner PW. In the case of a stepper (asopposed to a scanner) the mask table MT may be connected to ashort-stroke actuator only, or may be fixed. Mask MA and substrate W maybe aligned using mask alignment marks M1, M2 and substrate alignmentmarks P1, P2. Although the substrate alignment marks as illustratedoccupy dedicated target portions, they may be located in spaces betweentarget portions (these are known as scribe-lane alignment marks).Similarly, in situations in which more than one die is provided on themask MA, the mask alignment marks may be located between the dies.

A controller 500 controls the overall operations of the lithographicapparatus and in particular performs an operation process describedfurther below. Controller 500 can be embodied as a suitably-programmedgeneral purpose computer comprising a central processing unit, volatileand non-volatile storage means, one or more input and output devicessuch as a keyboard and screen, one or more network connections and oneor more interfaces to the various parts of the lithographic apparatus.It will be appreciated that a one-to-one relationship betweencontrolling computer and lithographic apparatus is not necessary. Onecomputer can control multiple lithographic apparatuses. Multiplenetworked computers can be used to control one lithographic apparatus.The controller 500 may also be configured to control one or moreassociated process devices and substrate handling devices in a lithocellor cluster of which the lithographic apparatus forms a part. Thecontroller 500 can also be configured to be subordinate to a supervisorycontrol system of a lithocell or cluster and/or an overall controlsystem of a fab.

A download station 600, described further below, is provided as part ofthe lithographic apparatus or as a separate device elsewhere in the fab,perhaps close to the lithographic apparatus or at a central location.The download station is connected to controller 500, a supervisorycontrol system and/or the overall control system. The download stationcan incorporate a computer system programmed to analyze the informationobtained from the inspection substrate, or such analysis can beperformed elsewhere.

Arrangements for providing liquid between a final lens element of theprojection system PS and the substrate can be classed into three generalcategories. These are the bath type arrangement, the so-called localizedimmersion systems and the all-wet immersion systems. The presentinvention relates particularly to the localized immersion systems.

In an arrangement which has been proposed for a localized immersionsystem, a liquid confinement structure 12 extends along at least a partof a boundary of an immersion space between the final lens element ofthe projection system PS and the facing surface of the stage or tablefacing the projection system. The facing surface of the table isreferred to as such because the table is moved during use and is rarelystationary. Generally, the facing surface of the table is a surface of asubstrate W, substrate table 60 which surrounds the substrate or both.Such an arrangement is illustrated in FIG. 2. The arrangementillustrated in FIG. 2 and described below may be applied to thelithographic apparatus described above and illustrated in FIG. 1.

FIG. 2 schematically depicts the liquid confinement structure 12. Theliquid confinement structure 12 extends along at least a part of aboundary of the immersion space 10 between the final lens element 100 ofthe projection system PS and the substrate table 60 or substrate W. Inan embodiment, a seal is formed between the liquid confinement structure12 and the surface of the substrate W/substrate table 60. The seal maybe a contactless seal such as a gas seal (such a system with a gas sealis disclosed in European patent application publication no.EP-A-1,420,298) or a liquid seal.

The liquid confinement structure 12 is configured to supply and confineimmersion liquid to the immersion space 10. Liquid is brought into theimmersion space 10 by liquid inlet 13. The liquid may be removed byliquid outlet 13.

The liquid may be contained in the immersion space 10 by a gas seal 16which, during use, is formed between the bottom of the liquidconfinement structure 12 and the facing surface of the table (i.e. thesurface of the substrate W and/or the surface of the substrate table60). The gas in the gas seal 16 is provided under pressure via inlet 15to a gap between the liquid confinement structure 12 and substrate Wand/or substrate table 60. The gas is extracted via a channel associatedwith outlet 14. The overpressure on the gas inlet 15, vacuum level onthe outlet 14 and geometry of the gap are arranged so that there is ahigh-velocity gas flow inwardly that confines the liquid. The force ofthe gas on the liquid between the liquid confinement structure 12 andthe substrate W and/or substrate table 60 contains the liquid in theimmersion space 10. Such a system is disclosed in United States patentapplication publication no. US 2004-0207824. Other liquid confinementsystems 12 can be used with the present invention.

FIGS. 2 and 3 show different features which may be present in variationsof confinement structure 12. The designs may share some of the samefeatures as FIG. 2 unless described differently. The features describedherein may be selected individually or in combination as shown or asrequired.

FIG. 2 shows a confinement structure 12 around the bottom surface of alast lens element. The last lens element 100 has an invertedfrustro-conical shape. The frustro-conical shape having a planar bottomsurface and a conical surface. The frustro-conical shape protrudes froma planar surface and having a bottom planar surface. The bottom planarsurface is the optically active portion of the bottom surface of thelast lens element, through which the projection beam may pass. Theconfinement structure surrounds at least part of the frustro-conicalshape. The confinement structure has an inner-surface which facestowards the conical surface of the frustro-conical shape. Theinner-surface and the conical surface have complementary shape. A topsurface of the confinement structure is substantially planar. Theconfinement structure may fit around the frustro-conical shape of thelast lens element. A bottom surface of the liquid confinement structureis substantially planar and in use the bottom surface may be parallelwith the facing surface of the table and/or wafer. The distance betweenthe bottom surface and the facing surface may be in the range of 30 to500 micrometers, desirably in the range of 80 to 200 micrometers.

The confinement structure extends closer to the facing surface of thewafer and wafer table than the last lens element. A space is thereforedefined between the inner surface of the confinement structure, theplanar surface of the frustro-conical portion and the facing surface.During use, the space is filled with liquid. The liquid fills at leastpart of a buffer space between the complementary surfaces between lensand the confinement structure, in an embodiment at least part of thespace between the complementary inner-surface and the conical surface.

Liquid is supplied to the space through an opening formed in the surfaceof the liquid confinement structure. The liquid may be supplied througha supply opening 20 in the inner-surface of the liquid confinementstructure. Alternatively or additionally, the liquid is supplied from anunder supply opening 23 formed in the undersurface of the confinementstructure 12. The under supply opening may surround the path of theprojection beam and it may be formed of a series of openings in anarray. The liquid is supplied to fill the space 10 so that flow throughthe space under the projection system is laminar. The supply of liquidfrom the opening 23 under the liquid confinement structure additionallyprevents the ingress of bubbles into the space. This supply of liquidfunctions as a liquid seal.

The liquid may be recovered from a recovery opening 21 formed in theinner-surface. The recovery of the liquid through the recovery openingmay be by application of an under pressure; the recovery through theopening 21 as a consequence of the velocity of the liquid flow throughthe space; or the recovery may be as a consequence of both. The recoveryopening 21 may be located on the opposite side of the supply opening 20,when viewed in plan. Additionally or alternatively, the liquid may berecovered through an overflow opening 24 located on the top surface ofthe liquid confinement structure 12.

Additionally or alternatively, liquid may be recovered from under theliquid confinement structure through a bottom recovery opening 25, 32.The bottom recovery opening may serve to hold (or ‘pin’) a liquidmeniscus to the liquid confinement structure. The meniscus forms betweenthe liquid confinement structure and the facing surface and it serves asborder between the liquid space and the gaseous external environment.The bottom recovery opening 25 may be a porous plate 25 which mayrecover the liquid in a single phase flow. The bottom recovery openingmay be a series of pining openings 32 through which the liquid isrecovered. The pining openings may recover the liquid in a two phaseflow.

Optionally radially outward, with respect to the inner-surface of theliquid confinement structure, is an gas knife opening 26. Gas may besupplied through the gas knife opening at elevated speed to assistconfinement of the immersion liquid in the space. The supplied gas maybe humidified and it may contain carbon dioxide. The supplied gas mayconsist essentially of carbon dioxide and water vapor. Radially outwardof the gas knife opening is a gas recovery opening 28 for recovering thegas supplied through the gas knife.

Features shown in FIG. 3 which are common to FIG. 2 share the samereference numbers. The confinement structure 12 has an inner surfacewhich complements the conical surface of the frustro-conical shape. Theundersurface of the confinement structure is closer to the facingsurface than the bottom planar surface of the frustro-conical shape.

Liquid is supplied to the space through supply openings formed in theinner surface of the confinement structure. The supply openings 34 arelocated towards the bottom of the inner surface, perhaps below thebottom surface of the fustro-conical shape. The supply openings arelocated inner surface, space apart around the path of the projectionbeam.

Liquid is recovered from the space 10 through recovery openings 25 inthe undersurface of the liquid confinement structure 12. As the facingsurface moves under the confinement structure, the meniscus may migrateover the surface of the recovery opening 25 in the same direction as themovement of the facing surface. The recovery openings may be formed of aporous member. The liquid may be recovered in single phase. In anembodiment the liquid is recovered in a two phase flow. The two phaseflow is received in a chamber 35 within the liquid confinement structure12 where it is separated into liquid and gas. The liquid and gas arerecovered through separate channels 36, 38 from the chamber 35.

An inner periphery 39 of the undersurface of confinement structureextends into the space away from the inner surface to form a plate 40.The inner periphery forms a small aperture which may be sized to matchthe shape and size of the projection beam. The plate may serve toisolate liquid either side of it. The supplied liquid flows inwardstowards the aperture, through the inner aperture and then under theplate radially outwardly towards the surrounding recovery openings 25.

In an embodiment the confinement structure may be in two parts: an innerpart 12 a and an outer part 12 b. For convenience this arrangement isshown in the right-hand part of FIG. 3. The two parts may moverelatively to each other, in a plane parallel to facing surface. Theinner part may have the supply openings 34 and it may have the overflowrecovery 24. The outer part 12 b may have the plate 40 and the recoveryopening 25. The inner part may have an intermediate recovery 42 forrecovering liquid which flows between the two parts.

Contamination of various types can adversely affect the performance of afluid handling system in a lithographic apparatus. Although theenvironment of the lithographic apparatus is kept to a very lowcontaminant level and the immersion liquid, e.g. water, is very pure,the possibility of particulate contamination of the fluid handlingsystem cannot be wholly eradicated. The presence of even smallcontaminants at critical locations within the fluid handling system canreduce its effectiveness.

For example, the presence of a fiber on, for example adhered to, thelower surface of a liquid confinement structure 12 may increasedefectivity and may contribute to a reduction in productivity. Thepresence of a fiber adjacent, or even over, a water extraction orificecan lead to additional water loss onto a production substrate duringexposures. Also, a partial or complete blockage of a gas outlet formingpart of a gas seal for confining the immersion liquid can lead to waterloss onto a production substrate. Water loss on a production substratecan cause defects in exposed patterns. The defects may be formed throughthe creation of watermarks on the resist as a consequence of evaporatingdroplets. In a different mechanism, a bubble may be generated oncollision between the meniscus of the confined immersion liquid and adroplet remaining on the substrate. The bubble may travel in theimmersion space to interfere with the path of the projection beam.

It is often difficult to detect that contamination has reduced theeffectiveness of the liquid confinement system. Often the first sign ofcontamination of a confinement structure 12 will be a decrease in yielddue to an increase in the number of defects in exposed patterns; therisk of an increase in defect count may not become immediately apparent.Opening the lithographic apparatus to inspect the liquid confinementstructure for contaminants is time consuming. The procedure ofinspection presents a risk of contamination, so it is undesirable toperform such an inspection unless absolutely necessary.

The present invention proposes an inspection substrate that can beloaded into the lithographic apparatus as if it were a productionsubstrate to be exposed. The inspection substrate is interchangeablewith a production substrate. The inspection substrate is compatible withthe lithographic apparatus. The inspection substrate contains one ormore image sensors that are configured to inspect a component, bycapturing images of the component, of the lithographic apparatus. Theimages are captured to enable contamination or damage, for example, ofthe component to be detected.

During inspection, which may be during operation of the lithographicapparatus, the inspection substrate is adjacent, or proximate to thecomponent. With respect to the inspection substrate, the component maybe visible from the path through the apparatus of a normal with respectto the upper surface of a production substrate (and therefore theinspection substrate). The component may be a functional subsystem ofthe lithographic apparatus, such as a liquid confinement system, or apart of a functional subsystem. The inspection substrate of the presentinvention can also contain further sensors, such as electronic sensors,which make measurements of a parameter of component. Parameters whichcan be measured include: surface topology, contamination or damage.Other parameters include operational status and temperature.

The images, and optionally other measurements, are then stored in memoryintegrated into the inspection substrate. It should therefore becontrasted with a test substrate on which test exposures are carried outin order to characterize the performance of the lithographic apparatus.(Such a test substrate may be referred to as a ‘witness substrate’ inthat the substrate takes the place of an exposure substrate for testingpurposes. The witness substrate may be coated with typical substratecoatings, such as photosensitive resist, so that tests which may involvetest exposures may be carried out to gain information regarding theprocesses applied to exposure substrates; as if the witness substratewas an exposure substrate.)

The image sensors of the inspection substrate of the present inventioncapture images of a component, or part of a component, at short range,e.g. with a range of less than 1 to 10 mm Sensing substrates withintegrated image sensors for detecting the projection beam are known butare only capable of measuring the projection beam and not otherfunctional subsystems. Since the inspection substrate of the presentinvention does not measure the projection beam it need not be capable ofwithstanding DUV radiation. In the absence of exposure to DUV exposurelight, the risks to DUV radiation on the lifetime on the inspectionsubstrate are minimal.

An inspection substrate according to an embodiment of the presentinvention is particularly useful in inspecting a liquid confinementsystem, especially a liquid confinement structure. A feature of theliquid confinement structure which the inspection substrate may be usedto inspect is the undersurface of the liquid confinement structure.Inspection of features present in the undersurface such as the openingsfor the passage of liquid and gas can be achieved using the inspectionsubstrate of the present invention.

FIG. 4 depicts the underside of a liquid confinement structure 12 in alithographic apparatus. The lower surface of the liquid confinementstructure 12, that is the surface which faces the substrate W duringoperation of the lithographic apparatus, is provided with severalgenerally parallel rows of apertures. The rows arranged may be generallyarranged concentrically around the immersions space. As described withreference to FIGS. 2 and 3, they may be used to help confine theimmersion liquid to the immersion space 11. These apertures may include(in a non-limited list) gas seal apertures 151, liquid extractionapertures 121 and liquid supply apertures 122.

The gas seal apertures 151 are supplied, when operating, with gas at ahigh pressure so as to form a high pressure region between the liquidconfinement structure 12 and substrate W. The high pressure regionfunctions to confine the immersion liquid to the immersion space 11 andis referred to as a gas seal. The liquid extraction apertures 121 areconnected to a low pressure source and in use extract gas and/orimmersion liquid in a one or two phase flow. The liquid extractionapertures 121 can function as a meniscus pinning feature. Liquid supplyapertures 122 supply liquid to the immersion space, e.g. to replenishliquid removed through the liquid extraction apertures 121.

The total width a of the inspected object, e.g. liquid confinementstructure 12, may be of the order of 4 to 40 mm, or larger. The variousapertures described may have different sizes, e.g. of the order of 10 μmto 1 mm Therefore, a small contaminant particle can easily block ordisrupt the flow around any of the apertures. If the contaminant is afiber, a single fiber could obstruct one or more openings along a row ofopenings. Detecting a contaminant particle therefore may requiremicroscopic inspection of the liquid confinement structure 12 which canbe very time consuming. It is often necessary to inspect the whole ofthe liquid confinement structure 12 undersurface if contamination issuspected since available information does not provide precisediagnostic information. For example, a higher of count of defects perwafer than expected might well not give any clue as to the number orlocations of any contaminants on the liquid confinement structure.

FIG. 5 depicts an inspection substrate IW according to an embodiment ofthe present invention. Inspection substrate IW comprises an inspectionbody that can be loaded into and transported by the lithographicapparatus. The inspection body may be made of the same material asproduction substrate. The body may have dimensions similar to, orsubstantially the same as, a production substrate. Therefore, theinspection substrate IW can be loaded into and handled by thelithographic apparatus in the same way as a production substrate.Inspection substrate body 200 may be a silicon wafer, e.g. of diameter300 mm or 450 mm.

Embedded in inspection substrate body 200 are: an imaging device 210; anilluminating device 220; a storage device 230; a controller andinterface 240; and a power storage device 250. The illuminating deviceand its arrangement are described further below with reference to FIGS.9 to 15. It will be appreciated that the lithographic apparatus is acomplex machine packed densely into a closed compartment, in some casesa vacuum chamber, which is not normally provided with any interiorillumination. Therefore, all necessary illumination for the imagingdevice has to be provided by the illuminating device.

These various components of the inspection substrate may bemanufactured, e.g. by use of lithographic techniques, directly onto asurface of the inspection substrate body 200. Additionally oralternatively the components can be separately formed and secured (e.g.by bonding or adhering) into place. A separate component can be securedin place in the inspection substrate body 200 in a recess in inspectionsubstrate body 200. Such a recess may be dimensioned to matchsubstantially the dimensions of the secured component. A cover plate,which is transparent at least in regions above the illuminating device220 and imaging device 210, can be provided. Desirably the variouscomponents of inspection substrate IW do not project out of either majorsurface of inspection substrate body 200. Where such a componentprojects from a major surface of the inspection substrate body, or thecomponent projects no more than is acceptable by the lithographicapparatus, for example no more than 20 micrometers or desirably less.

In the event that one or more components of the lithographic apparatusare not perfectly flush with an outer surface of inspection substratebody 200, an additional planarization layer (such as a coating) can beprovided to the respective outer surface to ensure that the relevantsurface matches the flatness specifications required by the lithographicapparatus.

Inspection substrate IW is desirably configured to be sealed against theinflow of the immersion liquid (e.g. waterproof) into the inspectionsubstrate body. Additionally or alternatively, the inspection substrateIW is desirably resistant to immersion liquid. For example, any gapsformed between a component in a recess of the inspection substrate bodyand the inspection substrate body are effectively sealed. Further, evenif such a gap were to form at such a recess, the inspection substrate IWwould be sufficiently resistant to the inflow of any liquid into such agap. For these reasons, the inspection substrate IW can be used withimmersion liquid present in the immersion space.

Imaging device 210 can be formed as a standard CMOS or CCD imagingsensor. The sensor may be provided with a microlens. The imaging device210 is described further below.

Illuminating device 220 can comprise one or more light emitting diodesor laser diodes. In an embodiment, illuminating device 220 comprises aplurality of white light emitting diodes—e.g. four, six oreight—disposed around imaging device 210. Illuminating device 220 may beconfigured to emit radiation of any convenient wavelength, e.g. in theinfrared, ultraviolet or visible ranges. Illuminating device 220desirably emits radiation that can be detected by imaging device 210 sothat a phosphorescent or scintillation layer is not required.Illuminating device 220 may be configured to emit radiation of awavelength or wavelengths in which expected forms of contaminationcontrast most strongly with the material of the object to be inspected.Desirably the radiation has a wavelength shorter than the diameters ofcontaminant particles to be detected. Illuminating device 220 can beconfigured to emit radiation of a polarization state in which theexpected forms of contamination contrast most strongly with the materialof the object to be inspected.

As described further below, illuminating device 220 is configured toprovide dark field illumination, i.e. the direction of illumination,which is determined by the position of the illuminating device 220relative to the target area and the image sensor 210, is selected sothat radiation that is specularly reflected at the target area does notreach the imaging device 210. Illuminating device 220 may be providedwith multiple illumination elements that emit different wavelengths orpolarization states and are separately controllable so as to allowimages to be taken under different illumination conditions.

Storage device 230 desirably takes the form of non-volatile memory suchas NAND flash memory. However, static or dynamic RAM may be used ifpower storage device 250 is capable of providing sufficient power forlong enough for the inspection substrate IW to be removed from thelithographic apparatus and data downloaded.

Controller and interface 240 controls the overall operations of theinspection substrate IW. The controller and interface 240 communicateswith external devices. Controller and interface 240 may comprise amicroprocessor, program memory and working memory. Additionally oralternatively, controller and interface 240 may use a section of storagedevice 230 to store a program to be executed and for making memory.Controller and interface 240 may be configured to communicate withdifferent components of the inspection substrate and externally to theinspection substrate IW, using a wired protocol (e.g. USB or IEEE 1394),or a wireless protocol (e.g. Wi-Fi™ or Bluetooth™), or both. In view ofhigh levels of electromagnetic noise often present in a lithographicapparatus wireless communication with the inspection substrate mightonly be effective when the inspection substrate is outside thelithographic apparatus.

Power storage device 250 may be a battery. Power storage device 250 isdesirably a secondary cell, such as a lithium-ion cell. Additionally oralternatively power storage device 250 can be a supercapacitor.

FIG. 5 depicts in cross section a part of the inspection substrate IW inuse to image a lower surface of immersion liquid confinement structure12. Imaging device 210 comprises an image sensor 211 that includes atwo-dimensional array of imaging elements 212. Above the two-dimensionalarray of imaging elements 212 is provided a filler layer 213, which mayinclude a spectral filter and/or a polarizing filter. A polarizingfilter can increase the contrast between diffuse reflections andspecular reflections. In the event that imaging device 210 is a colorimaging device, the spectral filter may be a Bayer filter array. Abovefilter layer 213 is provided microlens 214. Microlens 214 is configuredto form an image of the lower surface of a part of the lower surfaceimmersion liquid confinement structure 12 onto at least a part of thetwo-dimensional array of image sensing elements 212. The curvature ofmicrolens 214 is significantly exaggerated in FIG. 5.

The working distance between inspection substrate IW and the inspectedobject, e.g. a component of the lithographic apparatus such as the lowersurface of liquid confinement structure 12, is short. Because of theshort working distance it is difficult to form an image of a large area,or surface, of the inspected object using just a single microlens. Ifthe area that can be imaged at one time is insufficient, it is possibleto enable imaging of a larger area of the inspected object for exampleby taking multiple images whilst moving the inspection substrate IWshort distances (i.e. shifting the substrate stage 60 that holds theinspection substrate IW) between images. Other techniques to increasethe effective size of the area that can be imaged exist and some of themare described here. Some of them enable the size of the area imaged inone image to be increased. In an embodiment the inspected object, e.g.the lower surface of the liquid confinement structure, may be moved fromits normal position, e.g. raising the liquid confinement surface, toincrease the working distance between the inspection substrate IW andthe lower surface of the liquid confinement structure. Raising theliquid confinement structure enables a greater area of the surface ofthe lower surface of the liquid confinement structure to be imaged.

In an embodiment of the present invention, it may only be necessary toinspect a small part of the component and therefore the field of view ofimaging device 210 on the component, e.g. lower surface of liquidconfinement structure 12, can be less than about 5 mm², e.g. less than 1mm², e.g. about 0.4 mm×0.4 mm. The focal length of microlens 214 is lessthan about 5 mm, e.g. less than 1 mm. Microlens 214 may be a Fresnellens in order to provide a greater optical power in a given thickness orto reduce the thickness of the lens for a given optical power.

In an embodiment, the image of the part of liquid confinement structure12 is projected onto an array of pixels in the two-dimensional array ofimage sensing elements 212. The size of the array depends on the desiredimaging resolution and the desired field of view. Commercially availabledesigns of image sensor provide sufficient numbers of pixels to provideboth ample resolution and field of view. Desirably the imaging device iscapable of resolving features on the object to be inspected ofdimensions less than 100 μm, more desirably less than 10 μm, for exampleabout 1 μm. If imaging device 210 is a monochrome imaging device, theneach pixel may correspond to a single image sensing element. If imagingdevice is a color imaging device then each pixel may correspond to agroup of four image sensing elements. Imaging device 210 furthercomprises a bus 215 connecting it to readout circuit 216. The readoutcircuit 216 controls the readout process and performs pre-amplificationon the output signals which are then communicated to storage device 230.Even if only a part of the array of imaging elements 212 is used, it maybe economical to use a commercially available imaging device 210 that islarger than necessary rather than a custom designed device. This isbecause existing commercial masks and processes can be used to form theimaging device 210. Additional expense in developing a new design andcustom masks can be avoided.

In an embodiment of the invention, inspection substrate IW is providedwith a plurality of imaging devices 210 so as to enable the object to beinspected to be scanned more quickly. The arrangement of imaging devices210 on inspection substrate IW can be optimized to at least a part ofthe shape of the object to be imaged so as to most efficiently image it.If the imaging devices 210 form a pattern which matches the shape of theinspected object, then the imaging of the inspected object can be takenone image. If the pattern formed by the imaging devices formed on theinspection substrate IW corresponds to a part of a shape of theinspected object, a number of images would be made before the inspectionof the inspected object is complete.

For example if the imaging devices form a pattern corresponding to aside of a four sided shape formed by the openings in the underside ofthe liquid confinement structure, four images should be made beforeinspection of the confinement structure undersurface is completed. Thenumber of imaging devices 210 may be further minimized if the inspectionsubstrate IW is moved whilst the imaging process is carried out.

If a liquid confinement structure having a different shape, e.g. roundor oval, a different arrangement of imaging devices can be used. If itis only desired to inspect a part of a liquid confinement structure,e.g. an opening or array of openings for fluid, the number andarrangement of the imaging devices can be adapted to the shape of thepart that is to be inspected rather than the whole liquid confinementstructure.

For example, in an embodiment of inspection substrate IW to image thecomplete lower surface of a liquid confinement structure 12 that has theshape in plan of a square frame oriented at 45° to x and y axes of theapparatus, an arrangement of image sensing devices 210 as shown in FIG.6 can be used. This arrangement comprises three image sensing elements210 each having a field of view having a dimension at least equal to thewidth of one side of the liquid confinement structure 12, or morespecifically, or in the alternative, the width of a row of openingsforming a pattern on the underside of the liquid confinement structure12. The image sensing devices 210-1, 210-2 and 210-3 are spaced apart bya distance b equal to the length of one side of the liquid confinementstructure. The length ‘b’ may more specifically, or in the alternative,be the separation between centers of the sides of the liquid confinementstructure 12. In use, the inspection substrate IW is orientated in thelithographic apparatus such that the two of the imaging devices, e.g.210-1, 210-2 can be located under a pair of adjoining corners of theshaped formed by the rows of opening in the undersurface of the liquidconfinement structure 12. Thus, an imaginary line joining two of theimaging devices, e.g. 210-1 and 210-2, is parallel to one side (e.g. afirst side) of the liquid confinement structure 12.

In the same orientation of the inspection substrate IW relative to thelithographic apparatus another pair of the imaging devices, e.g. 210-2,210-3 can be located under another pair of adjoining corners of theshaped formed by the rows of opening in the undersurface of the liquidconfinement structure 12. Thus, an imaginary line joining another two ofthe imaging devices, e.g. imaging devices 210-2 and 210-3, is parallelto another side (e.g. a second side) of liquid confinement structure 12.The first and second sides may be adjoining sides of the shape in theundersurface of the liquid confinement structure 12.

With such an arrangement and a liquid confinement structure of thatshape, two opposite sides of liquid confinement structure 12 can bescanned simultaneously with one movement of imaging wafer IW relative toliquid confinement structure 12. The other two opposing sides of liquidconfinement structure 12 can then be imaged with a single further scanof inspection substrate IW relative to liquid confinement structure 12.

Additional image sensing devices 210-n maybe located on the inspectionsubstrate IW. The additional image sensing devices 210-n may be spacedapart by a distance equal to a fraction of the distance b alongimaginary lines parallel to the sides of the liquid confinementstructure 12. Having additional image sensing devices 210-n can allow awhole side of the liquid confinement structure 1 to be imaged in ashorter scan.

The separation between the imaged object and the image sensor can be toosmall to allow a complete width of the area to be imaged to be formed asa single image on the image sensor 210. In that event the microlens 214can be arranged to form a staggered array of images of smaller parts ofthe imaged object as shown in FIG. 7. After scanning, the collectedimages can be processed to generate a single contiguous image of thedesired object.

As shown in FIG. 7 the microlens 214 of the image sensing device 210 isarranged in a two dimension array of elements, F₁ to F_(n). In theFigure an arrangement of microlens elements are shown in a formation oftwo staggered parallel rows; although any staggered formation patternmay have any number of rows which may be parallel. In the shownstaggered formation microlens elements F₁, F₂ and F₃ are arranged in afirst linear array, spaced apart so that there is a gap in the firstlinear array between adjoining microlens elements F₁, F₂ and F₃.Microlens elements F₄ and F₅ are in a second linear array spaced apartfrom the first linear array. In the second linear array, the microlenselements F₄ and F₅ are spaced apart from each with a similar spacingbetween adjoining micro lens elements as the microlens elements F₁, F₂and F₃. The first and second linear arrays may be substantially straightand may be substantially parallel. The first and second linear arraysdiffer in that the microlens elements of the second linear array arealigned with the gaps of the first linear array. Therefore, a microlenselement of at least of one the first and second linear arrays isintersected in a direction orthogonal to the alignment of the first andsecond linear arrays.

A third linear array of microlens elements F_(n) may be spaced apartfrom the second linear array. The third linear array may take theformation of the first linear array. Although FIG. 7 does not show afourth linear array, if a fourth linear array is present it would takethe formation of the second linear array. Thus successive linear arraysof microlens elements take the staggered formation of the first andsecond linear arrays.

FIG. 7 also depicts features of the undersurface of the liquidconfinement structure 12: an outer edge 19 of the undersurface of theliquid confinement structure, a linear array of gas openings 15, and alinear array of liquid supply openings 122. Although these features of aliquid confinement structure are explicitly depicted, they are intendedto correspond to any features, such as openings, present in the surfaceof the inspected object. Note that the outer edge 19 and an inner limit20 represent the broadest extent of the detectable features on theinspected object. For example for the shape of the openings in theundersurface of a liquid confinement structure, outer edge 19 and innerlimit 20 represent the displacement of radially furthest feature andradially furthest feature, respectively from the path of the projectionbeam. This may be useful to ensure the full side of the feature of theinspected object is imaged, for example where the side is curved and notstraight. See for example the footprint of the liquid confinementstructure disclosed in EP 2131241 A2 (which is hereby incorporated byreference in its entirety) which has curved sides.

As shown in FIG. 7, the depicted features 15, 122 are shown in lineararrays aligned at an angle, desirably orthogonal to the linear arrays ofthe microlens elements. Therefore in moving the imaging substrate in adirection orthogonal to the alignment of the linear arrays enables allthe features of the inspected object to be imaged.

FIG. 8 depicts a method of use of the inspection substrate IW to inspectinternal functional subsystems, such as the liquid confinement system,of the lithographic apparatus without opening the lithographicapparatus. Therefore, the downtime required for inspection is greatlyreduced and the risk of further contamination avoided. Inspectionsubstrate IW is loaded S1 into the lithographic apparatus in exactly thesame way as a resist-coated substrate (or production substrate) isloaded for exposure. Inspection substrate IW is placed onto substratetable 60 by a substrate handler.

Once loaded into the lithographic apparatus and placed on the substratetable 60, inspection substrate IW may be subjected to certainprequalification steps S2, e.g. flatness measurements, to validate theinspection substrate IW and verify that it will not damage thelithographic apparatus. However a complete pre-characterization andtemperature conditioning process as normally performed for productionsubstrates need not be applied.

After any initial steps, the inspection substrate IW is positioned bysubstrate table 60 so that the imaging device 210 is positionedunderneath and facing the functional subsystem to be inspected, e.g. thelower surface of immersion liquid confinement structure 12. Inpositioning the inspection substrate on the substrate table 60, theinspection substrate is orientated in a preferred direction, for exampleso that sensor devices on the substrate are appropriately orientatedwith respect to features of the inspected object, such as the openingsin the undersurface of the liquid confinement structure 12. In alithographic apparatus with separate measurement and exposure stations,this may involve a transfer S3 of the inspection substrate IW to theexposure station. Illuminating device 220 is turned on to illuminate theobject to be inspected and imaging device 210 captures images S4 of theobject to be inspected. Alternatively or in addition, pressuremeasurements from pressure sensor 260 are recorded. Captured images,which may be in the form of a sequence of still images, or a movingimage, and/or pressure measurements are stored in storage device 230.Inspection substrate IW can be stepped or scanned S5 underneath theobject to be inspected during the inspection process so as to takeimages of different parts of the object to be inspected.

During the inspection process the liquid confinement system may beinoperative, partially operative or wholly operative. For example, theliquid confinement system may be inoperative and no immersion liquidpresent during an inspection using imaging device 210 so that unobscuredimages of the lower surface of the liquid confinement structure 12 canbe obtained. When an inspection process involving pressure measurementsis performed, it is desirable that the gas seal and other gas suppliesand extraction systems are operative but immersion liquid may or may notbe present.

If the inspection substrate IW is resistant to the immersion liquid thenan inspection using images can be carried out whilst immersion liquid ispresent and confined by the liquid confinement system. Such aninspection may not provide clear images of a liquid confinementstructure 12 but may instead provide useful images showing the behaviorof immersion liquid in the apparatus.

Once all desired images and/or measurements have been collected,inspection substrate IW is unloaded S7 from the apparatus in the sameway as a production substrate. However rather than being sent to a trackfor processing, inspection substrate IW is transferred S8 to a downloadstation 600. At the download station 600 data of the stored imagesand/or measurements can be downloaded S9 from storage device 230 viacontrol system and interface 240. Control system and interface 240 mayconnect to the download station via a wireless communication technique,such as Wi-Fi™ or Bluetooth™. Power storage device 250 can be rechargedat the download station, e.g. via a wireless induction charging system.Alternatively, the lower surface of inspection substrate IW can beprovided with electrical contacts for both downloading of data of imagesand/or measurements from storage device 230 and for charging powerstorage device 250.

The downloaded data is then analyzed S10 to identify any faults orproblems with the object that has been inspected, for examplecontamination (such as a blocked opening) in the case of theundersurface of a liquid confinement structure 12. Analysis of thedownloaded data can be manual, automatic or a combination of manual andautomatic processes. Automatic analysis may include pattern recognitionor comparison with reference data, e.g. images of a clean and properlyfunctioning object. If it is decided S11 that a problem exists thenremedial action S12 is taken. Remedial action to be taken will depend onthe detected problem. If contamination is detected, a cleaning processcan be performed. The cleaning process may require decommissioning andopening of the lithographic apparatus for a manual clean or anintegrated cleaning device can be used. In some circumstances, e.g. ablockage of a gas seal aperture 151, a flushing operation in whichliquid or gas is caused to flow in the opposite to normal direction canbe sufficient to remove the contaminant and rectify the problem. Aftercompletion of any remedial action, it is decided S13 whether are-inspection of the object is required and if so the process repeats.

An arrangement for illumination of the target area TA is shown in FIGS.9 to 11 which are enlarged cross-sectional views of a part of theinspection substrate IW. Illumination devices 220, for example LEDs, arelocated in illuminator recesses 2002 within the body 200 of inspectionsubstrate IW. Only one illumination device 220 is shown in FIG. 9 butinspection substrate IW has a plurality of, e.g. four, illuminatingdevices located symmetrically around imaging device 210. Imaging device210, e.g. a CCD image sensor, is located in an imager recess 2003 withinbody 200. Imaging device 210 is located centrally among the illuminationdevices 220.

A transparent plate 2001, e.g. made of glass, is attached to the surfaceof body 200 covering illumination devices 220 and imaging device 210.Transparent plate 2001 has flat and parallel inner (lower in thefigures) and outer (upper in the figures) surfaces except for a regionin front of imaging device 210 where the inner and/or outer surfaces areshaped to form microlens 214. In an embodiment, just the inner surfaceis shaped to form microlens 214; the outer surface is flat. Microlens214 can also formed as a separate element which is adhered to the innersurface of transparent plate 2001.

Illumination devices 220 emit radiation over a wide range of angles.FIGS. 9 to 11 depict selected rays emitted by one of the illuminationdevices 220.

In particular, FIG. 9 depicts the rays that provide the majority ofillumination of the target area TA. Ray bundle R1 exemplifies radiationthat propagates directly (i.e. without undergoing any reflections) fromthe illumination device 220 to target area TA. Rays in ray bundle R1 arerefracted by the transparent plate 2001 but not reflected by anysurfaces before the target area TA. It will be seen that rays of raybundle R1 that are specularly reflected in the target area TA will notenter microlens 214 and hence not reach imaging device 210. This isachieved by selecting the distance between illumination device 220 andimaging device 210, and hence the angle at which direct illuminationarrives at the target area TA, according to the working distance betweenthe inspection substrate IW and the component being inspected. However,if radiation is scattered, e.g. by contamination, in target area TA,some of the scattered radiation will be captured by microlens 214 andfocused on imaging device 210.

Ray bundle R2 exemplifies radiation that reaches the target area TAafter having been reflected by the lower surface of the component beinginspected, e.g. liquid confinement structure 12, and the outer (upper)surface of transparent plate 2001. Again, the relative positions ofillumination device 220, imaging device 210 and component beinginspected determine which rays leaving illumination device 220 reach thetarget area after two reflections. In an embodiment, as depicted, theradiation of ray bundle R2 arrives at target area TA at angles such thatradiation specularly reflected at target area TA will not be captured bymicrolens 214 and not reach imaging sensor 210. The radiation of raybundle R2 therefore provides a useful contribution to dark-fieldillumination of target area TA. In an embodiment, a reflective coatingis selectively provided on the outer surface of transparent plate 2001to increase the reflection of radiation of ray bundle R2 so as toincrease the intensity of the illumination of the target area TA.

FIG. 10 depicts some radiation emitted by illumination device 220 thatcould reach the imaging device 210 without having illuminated the targetarea TA. This radiation, exemplified by ray bundle R3, is reflected bythe lower surface of the liquid confinement structure 12 (or othercomponent to be inspected) into microlens 214 and then focused on theimaging device 210. This radiation is undesirable because it does notcontribute to an image of the target area TA. The radiation of raybundle R3 can be partially suppressed by provision of a localized opaquecoating on the outer surface of transparent plate 2001 in the regionwhere ray bundle R3 first passes through transparent plate 2001.However, since illumination device 220 is not a point source, an opaquecoating might also block some desirable radiation, e.g. from ray bundlesR1 and R2. Therefore, the size and location of an opaque coating toblock undesired radiation must be chosen as a compromise betweensuppressing undesired radiation and not suppressing desired radiation.

FIG. 11 also depicts some undesirable radiation that could reach theimaging device without usefully contributing to illumination of thetarget area TA. In this case, radiation in ray bundle R4 reflects offthe lower surface of the liquid confinement structure 12, then off theouter surface of transparent plate 2001 and then again off the lowersurface of liquid confinement structure 12. Thus, radiation in raybundle R4 can be suppressed by a localized absorbing, i.e.non-reflective, coating provided in the region where the radiation ofray bundle R4 reflects off upper surface of transparent plate 2001. Thisregion may overlap with regions of the transparent plate which reflectdesirable radiation, e.g. ray bundle R2, or through which desirableradiation passes, e.g. ray bundle R1. In that case, the size andlocation of a non-reflective coating must be chosen as a compromisebetween suppressing undesired radiation and not suppressing desiredradiation.

As depicted in all of FIGS. 9 to 11, radiation emitted by illuminationdevice 220 can reflect off the walls of the illuminator recess 2002and/or imager recess 2003. Generally radiation reflected off the wallsof the illuminator recess 2002 and/or imager recess 2003 is undesirableand does not contribute to dark-field illumination of the target area.Therefore, in an embodiment non-reflective coatings are provided onwalls of the illuminator recess 2002 and/or imager recess 2003.

FIG. 12 depicts a simulated image captured by the imaging device of asquare object placed in the center of the target area TA. Intensitiesare shown on a logarithmic scale. The square object scatters radiationand shows up as a central bright region in the image, surrounded by arelatively dark field. The central bright region is formed by theradiation depicted in FIG. 9. The central bright region is an image ofthe target region and would have a different appearance if a differentobject, or no object, were present in the target region. Also shown areperipheral bright regions at the edges of the simulated image which areformed by the radiation depicted in FIGS. 10 and 11. The peripheralbright regions contain no information about the target region and aretherefore undesirable.

Because they are well separated from the central image of the targetregion, several approaches are possible to deal with the peripheralbright regions. For example, an opaque border can be applied to theimaging device to mask parts of the image sensor that do not image thetarget region. A smaller image sensor can be used. The peripheral partsof the image can be discarded during or after readout.

FIG. 13 depicts the imaging device 210 and illuminators 220 in cut-awayperspective view to show how the target area is illuminated frommultiple directions. Four illumination devices 220 are shown in thisFigure but more or fewer can be provided. If the illumination devices220 are evenly and symmetrically disposed around imaging device 210 thenthe imaging device will image contamination just as well whatever theorientation of the contamination. If contamination of a particularorientation is expected or most damaging then a non-uniform illuminationmay be adopted.

FIG. 14 depicts in cross section the transparent plate 2001 in greaterdetail. As mentioned, a part of the inner and/or outer surfaces oftransparent plate 2001 are shaped to form microlens 214. Opaque coatings2141 and 2142 are disposed on the outer and inner surfaces of thetransparent plate 2001 around microlens 214. Opaque coatings 2141 and2142 each have a central aperture so as to define a double stop to limitundesirable radiation, e.g. radiation not scattered from the targetarea, reaching imaging device 210. An anti-reflective coating may beprovided on the exposed outer surface of microlens 214.

A reflective coating 2143 is provided at one or more specific areas ofthe outer surface of transparent plate 2001 so as to reflect radiationtoward the target area TA. The location of reflective coating 2143 isselected so that the radiation it deflects toward target area TA wouldnot, if specularly reflected in the target area TA, reach imaging device210. Reflective coating can be formed by any convenient means, e.g.deposition through a lithographically patterned hardmask. Reflectivecoating may reflect a wide range of wavelengths or only selectedwavelengths as are emitted by the illumination device or are most usefulfor imaging contaminants. Reflective coating may be or may include asurface treatment, e.g. polishing.

An absorbing coating 2144 is provided at one or more specific areas ofthe outer surface of transparent plate 2001 so as to absorb radiationthat, if not absorbed, would be reflected toward the target area TA atsuch an angle that it would then be specularly reflected toward imagingdevice 210. Absorbing coating 2144 can be formed by any convenientmeans, e.g. deposition through a lithographically patterned hardmask.Absorbing coating 2144 may absorb a wide range of wavelengths or onlyselected wavelengths as are emitted by the illumination device or aremost useful for imaging contaminants. Absorbing coating 2144 may be ormay include a surface treatment, e.g. surface roughening. In somecircumstances it may be sufficient that absorbing coating 2144 scattersincident radiation rather than absorbs it.

Opaque coatings 2141 and 2142 form a double stop. As shown in FIG. 15, adouble stop limits the range of angles of radiation that can reach theimaging device. With only a single stop, radiation in the range ofangles indicated by the dashed lines would reach imaging device 210.With a double stop only radiation within the narrower range of anglesindicated by the solid lines can reach the imaging device 210.

In an embodiment of the present invention, the inspection substrate isused with a lithographic apparatus which has not been designed with theinspection substrate in mind. The lithographic apparatus may have nospecific provided means to communicate with or control the inspectionsubstrate when it is in lithographic apparatus. Therefore, theinspection substrate desirably operates autonomously. In an embodimentof the present invention, the inspection substrate is configured tocapture images and/or record measurements as soon as it is switched onprior to loading into the lithographic apparatus. The inspectionsubstrate may continue to capture images and/or record measurementsuntil it is unloaded and connected to the download station 600. This mayhowever require a storage device 230 with a very large capacity or mayrequire the sampling rate and/or resolution of captured images to belimited.

In an embodiment, the inspection substrate is programmed to captureimages or record measurements for specific time periods which may bedefined relative to an included clock or an initiating event. The timeperiods for image capture and/or measurement recording are predeterminedto match the timings of a predetermined program of movements of theinspection substrate through the lithographic apparatus.

In an embodiment, the inspection substrate is configured to determinewhen it is correctly located to begin capturing images and/ormeasurements. For example, the controller 240 can be configured tomonitor the image detected by the imaging device 210. Other sensors canbe provided to enable the inspection substrate to determine its locationwithin the lithographic apparatus. For example a MEMS sensor, e.g. anacceleration sensor, can be provided in the inspection substrate todetect movements of the inspection substrate IW and so synchronizemeasurements and/or imaging with the detected movements.

In an embodiment, the lithographic apparatus is provided with acommunication device for communicating with the inspection substratewhen the inspection substrate is loaded on the substrate table. Thecommunication means may be a wireless communication means, e.g. Wi-Fi™or Bluetooth™ or a wired connection via the underside of the inspectionsubstrate. If a wired connection can be provided, power may also beprovided to the inspection substrate avoiding the need to provide apower storage device 250 in the inspection substrate. A communicationdevice can be retrofitted to an existing lithographic apparatus.

If a communication device is provided in the lithographic apparatus itcan be used to instruct the inspection substrate to begin capturingimages and/or other measurements. The communication device can be usedto download captured images and measurement data. In an embodiment, datacaptured by the inspection substrate is downloaded and analyzed inparallel with the scanning of the object to be inspected. This allowsremedial action, e.g. a flushing operation, to be carried outimmediately upon detection of a problem. A re-scan can then be performedminimizing the downtime required for an inspection.

Although the present invention has been described above in relation touse of the inspection substrate to inspect a functional subsystem of alithographic apparatus, the inspection substrate can also be used toinspect a functional subsystem of another apparatus, such as a metrologyapparatus. An inspection substrate according to an embodiment of thepresent invention can be used in a process device of the track providedthat the inspection substrate is capable of withstanding conditionsprevailing in the track, e.g. high temperatures and application ofmaterials such as coatings. An inspection substrate according to anembodiment can be used in a test bed or partial apparatus.

In an embodiment, there is provided an inspection substrate forinspecting a component of an apparatus for processing productionsubstrates, the inspection substrate comprising: a body havingdimensions similar to the production substrates so that the inspectionsubstrate is compatible with the apparatus; an illumination deviceembedded in the body, the illumination device configured to emitradiation toward a target area of the component of the apparatus; and animaging device embedded in the body, the imaging device configured todetect radiation scattered at the target area and generate an image fromthe detected radiation, wherein the illumination device is configured toemit the radiation such that radiation that is specularly reflected atthe target area does not contribute to the image generated by theimaging device.

In an embodiment, radiation that has arrived at the target area directlyfrom the illumination device and is specularly reflected thereat doesnot contribute to the image generated by the imaging device. In anembodiment, radiation that has arrived at the target area after havingbeen reflected at least once and is specularly reflected thereat doesnot contribute to the image generated by the imaging device. In anembodiment, the inspection substrate further comprises a stop configuredto prevent radiation emitted by the illumination device and specularlyreflected from a part of the component outside the target area fromreaching the imaging device. In an embodiment, the stop is a doublestop. In an embodiment, the inspection substrate further comprises atransparent plate attached to the body and covering the imaging device;and wherein the stop comprises a first opaque coating on an outersurface of the transparent plate and having a first aperture; and asecond opaque coating on an inner surface of the transparent plate andhaving a second aperture. In an embodiment, the inspection substratefurther comprises an absorbing coating on a part of a surface of theinspection substrate that opposes the component, the absorbing componentabsorbing radiation emitted by the illumination device. In anembodiment, the inspection substrate further comprises a transparentplate attached to the body and covering the imaging device and theillumination device, the absorbing coating being provided on an outersurface of the transparent plate. In an embodiment, the inspectionsubstrate further comprises a reflective coating on a part of a surfaceof the inspection substrate that opposes the component, the reflectivecoating being located so as to reflect radiation emitted by theillumination device to the target area. In an embodiment, the inspectionsubstrate further comprises a transparent plate attached to the body andcovering the imaging device and the illumination device, the reflectivecoating being provided on an outer surface of the transparent plate. Inan embodiment, at least one of the imaging device and the illuminationdevice are located in a recess in the body and at least a part of asurface of the recess has an absorbing coating thereon. In anembodiment, the illumination device comprises a plurality of lightemitting diodes or laser diodes disposed around the imaging device.

In an embodiment, there is provided a method of inspecting a componentof an apparatus for processing production substrates, the methodcomprising: loading into the apparatus an inspection substrate asdescribed herein; scanning the inspection substrate proximate thecomponent whilst operating the sensor to generate inspection informationrelating to a parameter of the component; and storing the inspectioninformation in the storage device.

In an embodiment, the apparatus is a lithographic apparatus. In anembodiment, the component is a fluid handling system, in particular aliquid confinement structure.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat-panel displays, liquid-crystal displays (LCDs), thin film magneticheads, etc. The skilled artisan will appreciate that, in the context ofsuch alternative applications, any use of the terms “wafer” or “die”herein may be considered as synonymous with the more general terms“substrate” or “target portion”, respectively. The substrate referred toherein may be processed, before or after exposure, in for example atrack (a tool that typically applies a layer of resist to a substrateand develops the exposed resist), a metrology tool and/or an inspectiontool. Where applicable, the disclosure herein may be applied to such andother substrate processing tools. Further, the substrate may beprocessed more than once, for example in order to create a multi-layerIC, so that the term substrate used herein may also refer to a substratethat already contains one or multiple processed layers.

Although specific reference may have been made above to the use ofembodiments of the invention in the context of optical lithography, itwill be appreciated that the invention may be used in otherapplications, for example imprint lithography, and where the contextallows, is not limited to optical lithography. In imprint lithography atopography in a patterning device defines the pattern created on asubstrate. The topography of the patterning device may be pressed into alayer of resist supplied to the substrate whereupon the resist is curedby applying electromagnetic radiation, heat, pressure or a combinationthereof. The patterning device is moved out of the resist leaving apattern in it after the resist is cured.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of or about 436, 405, 365, 248, 193, 157 or 126 nm).and extreme ultra-violet (EUV) radiation (e.g. having a wavelength inthe range of 5-20 nm), as well as particle beams, such as ion beams orelectron beams.

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, including refractivereflective, magnetic, electromagnetic and electrostatic opticalcomponents.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described.

Any controllers described herein may each or in combination be operablewhen the one or more computer programs are read by one or more computerprocessors located within at least one component of the lithographicapparatus. The controllers may each or in combination have any suitableconfiguration for receiving, processing, and sending signals. One ormore processors are configured to communicate with the at least one ofthe controllers. For example, each controller may include one or moreprocessors for executing the computer programs that includemachine-readable instructions for the methods described above. Thecontrollers may include data storage media for storing such computerprograms, and/or hardware to receive such media. So the controller(s)may operate according the machine readable instructions of one or morecomputer programs.

One or more embodiments of the invention may be applied to any immersionlithography apparatus, in particular, but not exclusively, those typesmentioned above and whether the immersion liquid is provided in the formof a bath, only on a localized surface area of the substrate, or isunconfined. In an unconfined arrangement, the immersion liquid may flowover the surface of the substrate and/or substrate table so thatsubstantially the entire uncovered surface of the substrate table and/orsubstrate is wetted. In such an unconfined immersion system, the liquidsupply system may not confine the immersion liquid or it may provide aproportion of immersion liquid confinement, but not substantiallycomplete confinement of the immersion liquid.

A liquid supply system as contemplated herein should be broadlyconstrued. In certain embodiments, it may be a mechanism or combinationof structures that provides an immersion liquid to a space between theprojection system and the substrate and/or substrate table. It maycomprise a combination of one or more structures, one or more fluidopenings including one or more liquid openings, one or more gas openingsor one or more openings for two phase flow. The openings may each be aninlet into the immersion space (or an outlet from a fluid handlingstructure) or an outlet out of the immersion space (or an inlet into thefluid handling structure). In an embodiment, a surface of the space maybe a portion of the substrate and/or substrate table, or a surface ofthe space may completely cover a surface of the substrate and/orsubstrate table, or the space may envelop the substrate and/or substratetable. The liquid supply system may optionally further include one ormore elements to control the position, quantity, quality, shape, flowrate or any other features of the immersion liquid.

The descriptions above are intended to be illustrative, not limiting.Thus, it will be apparent to one skilled in the art that modificationsmay be made to the invention as described without departing from thescope of the claims set out below.

The invention claimed is:
 1. An inspection substrate for inspecting acomponent of an apparatus for processing production substrates, theinspection substrate comprising: a body having dimensions similar to theproduction substrates so that the inspection substrate is compatiblewith the apparatus; an illumination device embedded in the body, theillumination device configured to emit radiation toward a target area ofthe component of the apparatus; and an imaging device embedded in thebody, the imaging device configured to detect radiation scattered at thetarget area and generate an image from the detected radiation, whereinthe illumination device is configured to emit the radiation such thatradiation that is specularly reflected at the target area does notcontribute to the image generated by the imaging device.
 2. Theinspection substrate according to claim 1, wherein radiation that hasarrived at the target area directly from the illumination device and isspecularly reflected thereat does not contribute to the image generatedby the imaging device.
 3. The inspection substrate according to claim 2,wherein radiation that has arrived at the target area after having beenreflected at least once and is specularly reflected thereat does notcontribute to the image generated by the imaging device.
 4. Theinspection substrate according to claim 1, further comprising a stopconfigured to prevent radiation emitted by the illumination device andspecularly reflected from a part of the component outside the targetarea from reaching the imaging device.
 5. The inspection substrateaccording to claim 4, wherein the stop is a double stop.
 6. Theinspection substrate according to claim 5, further comprising atransparent plate attached to the body and covering the imaging device,and wherein the stop comprises a first opaque coating on an outersurface of the transparent plate and having a first aperture, and asecond opaque coating on an inner surface of the transparent plate andhaving a second aperture.
 7. The inspection substrate according to claim1, further comprising an absorbing coating on a part of a surface of theinspection substrate that opposes the component, the absorbing coatingabsorbing radiation emitted by the illumination device.
 8. Theinspection substrate according to claim 7, further comprising atransparent plate attached to the body and covering the imaging deviceand the illumination device, the absorbing coating lacing provided on anouter surface of the transparent plate.
 9. The inspection substrateaccording to claim 1, further comprising a reflective coating on a partof a surface of the inspection substrate that opposes the component, thereflective coating located so as to reflect radiation emitted by theillumination device to the target area.
 10. The inspection substrateaccording to claim 9, further comprising a transparent plate attached tothe body and covering the imaging device and the illumination device,the reflective coating being provided on an outer surface of thetransparent plate.
 11. The inspection substrate according to claim 1,wherein the imaging device and/or the illumination device is located ina recess in the body and at least a part of a surface of the recess hasan absorbing coating thereon.
 12. The inspection substrate according toclaim 1, wherein the illumination device comprises a plurality of lightemitting diodes or laser diodes disposed around the imaging device. 13.A method of inspecting a component of an apparatus for processingproduction substrates, the method comprising: loading into the apparatusan inspection substrate comprising: a body having dimensions similar tothe production substrates so that the inspection substrate is compatiblewith the apparatus, an illumination device embedded in the body, theillumination device configured to emit radiation toward a target area ofthe component of the apparatus, and an imaging device embedded in thebody, the imaging device configured to detect radiation scattered at thetarget area and generate an image from the detected radiation, whereinthe illumination device is configured to emit the radiation such thatradiation that is specularly reflected at the target area does notcontribute to the image generated by the imaging device; scanning theinspection substrate proximate the component while operating the imagingdevice to generate inspection information relating to a parameter of thecomponent; and storing the inspection information in the storage device.14. The method according to claim 13, wherein the apparatus is alithographic apparatus.
 15. The method according to claim 14, whereinthe component is a fluid handling system.
 16. The method according toclaim 13, wherein radiation that has arrived at the target area directlyfrom the illumination device and is specularly reflected thereat doesnot contribute to the image generated by the imaging device.
 17. Themethod according to claim 13, further comprising preventing radiationemitted by the illumination device that is specularly reflected from apart of the component outside the target area from reaching the imagingdevice using a stop.
 18. The method according to claim 13, furthercomprising absorbing radiation, emitted by the illumination device, byan absorbing coating on a part of a surface of the inspection substratethat opposes the component.
 19. The method according to claim 13,further comprising reflecting radiation emitted by the illuminationdevice to the target area using a reflective coating on a part of asurface of the inspection substrate that opposes the component.
 20. Themethod according to claim 13, wherein the imaging device and/or theillumination device is located in a recess in the body and at least apart of a surface of the recess has an absorbing coating thereon.